fix: Disable code signing to avoid Windows symlink errors

- Add signAndEditExecutable: false to skip code signing on Windows
- Clear electron-builder winCodeSign cache before build
- Add helpful error messages for symlink permission issues

The winCodeSign package contains macOS symlinks that fail to extract
on Windows without admin privileges or Developer Mode enabled.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

client/package.json

@@ -45,7 +45,8 @@
           "arch": ["x64"]
         }
       ],
-      "icon": "assets/icon.ico"
+      "icon": "assets/icon.ico",
+      "signAndEditExecutable": false
     },
     "mac": {
       "target": [